1. Field of the Invention
The present invention relates to pneumatic apparatus, and, more particularly, to pneumatic apparatus having a reciprocating piston.
2. Description of the Prior Art
Pneumatic hammers typically utilize pressurized pneumatic fluids, such as pressurized air from an outside source, to drive a piston forward to impact a tool (such as a chisel) held within the hammer. Subsequently, pressurized pneumatic fluid drives the piston back to position the piston to again strike the tool. The piston reciprocates in this manner within a chamber of the hammer housing.
The piston typically divides the chamber into two subchambers, with one subchamber (often designated an "impact" subchamber) on one side of the piston and the other subchamber (or "retracting" subchamber) on the other side of the piston. Pressurized pneumatic fluid is supplied to the impact subchamber to drive the piston forward toward the tool. Generally, after the piston strikes the tool, pneumatic fluid is supplied to the retracting subchamber, thereby driving the piston back, while the pneumatic fluid within the impact chamber is allowed to exhaust through an exhaust port. Near the end of the piston's travel in the retracting direction, pneumatic fluid is resupplied to the impact subchamber and the pneumatic fluid within the retracting subchamber is allowed to exhaust, thus reversing the direction of the piston and causing it to again strike the tool. In this manner, a reciprocating motion of the piston is maintained.
A difficulty often encountered with pneumatic hammers is the tendency of the piston to "center" when attempting to start the hammer, especially when the hammer is held in a horizontal position. This problem occurs when the pneumatic hammer is unable to develop a sufficient pressure differential upon opposing faces of the piston dividing the impact and retracting subchambers during the start up phase. Consequently, the piston centers itself in the middle of the chamber and does not oscillate.
Prior attempts to alleviate the foregoing problem includes devices such as that shown in U.S. Pat. No. 3,785,248 to Bailey, in which pneumatic fluid pressure above that which is utilized during oscillation is supplied to one of the subchambers in order to start the piston oscillating. However, the devices described therein require an additional external conduit and external valve arrangement connecting the conduit to the pressurized fluid source to supply the additional pressurized fluid to the hammer. This can make a pneumatic hammer more difficult to connect to the source and more cumbersome to operate.